In many turbine engines of the type utilizing centrifugal compressors coupled to radial turbines, the compressor and the turbine wheel are located in back-to-back relationship for compactness. Usually, an annular, narrow space exists between the two for thermal isolation purposes. That is, the space is provided to prevent undue quantities of heat from being transmitted from the turbine wheel to the compressor as a result of heating of the turbine wheel by hot gases of combustion.
While the space achieves such an objective, it presents a difficulty in that it must be sealed to prevent the flow of gas between the compressor side of the machine and the turbine side of the machine through such space at the interface between the rotor and the stator of the machine. Further, the sealing of such space should be such that the seal itself does not transmit unduly large quantities of heat from the turbine side of the engine to the compressor side.
To solve this difficulty, it has been conventional to provide an annular seal made up of two components. A first component is a forward seal plate which is secured by any suitable means to the engine stator on the compressor side thereof and which extends into the space between the compressor and the turbine into almost touching relation to the boundary of the space at its radially inner extremity. This seal plate holds down passage of gas from the compressor side to the turbine side of the engine to some desired amount (frequently, a small amount of gas passage is preferred to provide for some rotor cooling). However, it is not capable of preventing heat transfer from the turbine side of the engine to the compressor side.
In order to minimize such heat transfer, prior art seals additionally include a so-called diaphragm which is a relatively thin, ring-shaped piece of metal which is mounted on a forward seal plate near its radially outer periphery and extends radially inwardly therefrom to have its radially inner edge suitably mounted to the seal plate. The main body of the diaphragm is spaced from the seal plate thereby establishing an air pocket between the two which severely impedes heat transfer from the turbine side of the engine to the compressor side.
During engine operation, extremely high temperatures are generated at the turbine side of the engine. As a consequence, the seal plate and the diaphragm are subjected to thermal cycling and the diaphragm in particular experiences significant thermal growth in the process. Furthermore, there is a substantial thermal gradient radially across the seal assembly. These two factors result in distortion of the diaphragm during various operating conditions and will cause cracking leading to eventual failure. In order to prevent such distortion from resulting in interfering contact between the turbine wheel and the diaphragm, the clearance between the two must be kept relatively large. And, of course, utilizing a relatively large clearance increases the leakage flow path around the seal plate. The natural result is, of course, increased leakage and decreased operational efficiency of the machine.
Commonly assigned U.S. Pat. No. 4,932,207, issued Jun. 12, 1990 to Harris et al., discloses a highly desirable seal design which minimizes the clearance between the seal plate and turbine while providing an isolation space between the two to prevent heat transfer and accommodating the thermal cycling of the seal. The seal design minimizes to some extent the hot gas which enters the dead air space at the point where a three-layer laminate abuts an axial face on the forward seal plate and the housing support adjacent the annular nozzle. The present invention is directed at improving the insulating effects of the Harris et al. seal while continuing to minimize the amount of gas leakage.